Trypanosoma cruzi is a parasitic hemoflagellate and is the causative agent of Chagas Disease. This parasitic disease constitutes a major human health hazard in South and Central America, and, thus far, no successful chemotherapeutic cure or immunoprophylactic methods to prevent infection have been developed. Since both man and experimental animals can developed acquired resistance against acute infections of T. cruzi, the development of an effective vaccine for prevention and control of the disease should be feasible. This approach which we have taken for the development of a vaccine against this parasite focuses on the surface antigens of the infective form of the parasite. The rationale for this approach is based on the knowledge that interiorization of the parasite into the host cell is essential for establishing infection. The penetration process is a membrane-mediated phenomenon which likely involves parasite surface proteins. In accord with this line of reasoning, host cell penetration has been shown to involve at least one trypomastigote specific surface antigen of Mr 85, and are currently engaged in producing two of these antigens in amounts sufficient for immunization trials. In continuing these studies, the experiments that I propose for the next granting period are: (1) To test the ability of the 85, 90 and 34-41 kDa trypomastigote specific surface proteins to confer protective immunity to mice against an otherwise lethal inoculum of T. cruzi; (2) To determine whether the extent of antigenic diversity in the 85 kDa gene family among different clonal isolates of the parasite is sufficient to influence vaccine design; (3) To examine the possibility that spontaneous antigenic variation similar to that found in the African trypanosome occurs within the 85 kDa gene family in T. cruzi; (4) If substantial variation and/or diversity does occur within this gene family, we will determine whether constant and variable regions occur within the different expressed members of the gene family which might be exploited for vaccine design; (5) To continue work on the isolation, characterization and expression of the 34-41 kDa surface protein gene for subsequent immunization trials.